The main objective of this research is to characterize the mechanism by which human platelets are injured by infectious agents and immune complexes. The membrane receptor for immunoglobulin Fc fragment will be characterized using cellular fractionation, radioisotope, and immunochemical techniques. Staphylococcal Protein A will be used as a probe for Fc fragments determinants of IgG. Activation of human platelets by Histoplasma antigens will be studied. The role of complement and platelet-activating factors elaborated by lymphocytes will be analyzed. One further objective of this research is to establish the mechanism by which human platelets are influenced by antiinflammatory steroids.